Calculator



W. T. ALLEN June 22, 1948.

CALCULATOR 2 Sheets-Sheet 1 Filed April 1, 1943 INVENTOR )wzzace $60167 ATTORNEY w. T. ALLEN 2,443,882

CALCULATOR 2 Sheets-Sheet 2 June 22, 1948.

Filed April 1, 1943 wzzaei %m W A TTORNE Y H; MA

Patented June 22, 1948 CALCULATOR Wallace T. Allen, Deal-born, Mich., assignor to Square D Company, Detroit, Mich, a corporation of Michigan Application April 1, 1943, Serial No. 481,396

2 Claims.

This invention relates to calculators, and more particularly to a voltage drop, wire size calculator for determining percentage voltage drop and/or wire size for an electric wiring systerm.

In the past, various slide and rotary calculators have been used to aid in the determining of percentage voltage drop or wire size for wiring systems, but these calculators have been open to the objection that they were computed for a single value of power factor for the system and required the use of charted conversion factors to correct the results to the actual power factor of the system.

An object of the present invention is to provide a voltage drop, wire size calculator in which provision is made for a direct reading which shall I take into account the actual power factor of the circuit.

Another object of the invention is to provide a voltage drop, wire size calculator in which the six variables, system voltage, current, length of wire, power factor, voltage drop, and wire size may all be set or read directly from the calculator.

Other objects and features of the invention will be readily apparent to those skilled in the art from the specification and appended drawings illustrating certain preferred embodiments in which:

Figure 1 is a View of a slide type calculator according to the present invention.

Figure 2 is a view of a slightly modified form of calculator also of the slide type.

The calculator shown in Figure 1 involves a body portion l containing two slides 2 and 3. The body portion l comprises a flat stiff envelope within which the two flat stiii slides 2 and 3 are disposed so as to be longitudinally movable. In the face of the body portion l are three windows, 4, 5 and 6, within which portions of the slides and indicia thereon are visible. At the top and bottom edges of window 4 upon the body portion 1 are placed the indicia indicating the system voltage. .These are disposed on regular logarithmic scales with the upper set of indicia indicating single phase alternating current and direct current voltages and with the lower set of indicia indicating voltages for three phase systems. The upper portion of the slide 2 is provided with indicia, indicated at 1, representing the circuit amperes or the value of current which the circuit is designed to carry. These indicia are upon a logarithmic scale and are visible throughthe window 4 for purposes of setting the position of the slide 2. Upon the lower portion of the face of slide 2 are disposed indicia, indicated at 8, on a logarithmic scale, representing the distance one way in feet of the length of the circuit wiring. These are visible through the window 5. The slide 3, upon its face adjacent the upper edge, is provided with indicia indicated at 9 representing the percentage voltage drop of the circuit; these indicia again are on a logarithmic scale, are immediately opposite the indicia of distance in feet, and are visible through the window 5.

Adjacent to the edge of the window 6 are placed indicia indicated at H) representing the power factor of the circuit, again on a logarithmic scale and placed on the face of the body portion I. Upon the main body portion of the slide 3, and placed so as to be visible through the window 6 when disposed therebeneath, are curved lines representing the sizes of wires which may be used in the wiring system. These curves may be computed and drawn for any desired wiring size-for example, from No. 14 wire through a wire size of 500 MCM. These curves may be determined by computing the actual percentage voltage drop for the various wire sizes, at expected wire spacing and temperature, at the various power factors. The plotting of the values obtained, of percentage voltage drop against percentage power factor, will result in the plurality of curves for the different wire sizes.

The computations of the percentage voltage drop for various wire sizes at normal conduit wire spacing and temperature and at the various power factors may be carried out by various formula computation and the following is an example of one manner of computation.

The inductive reactance of the wires for vari-- ous sizes and at their normal conduit spacings is computed from the formula X equals 2 pi f (s0+714.1 log 10- Where small equals frequency, D equals distance between conductor centers, r equals radius of conductors in the same units as D; the logarithm being to the base It]. The resistance of the various size wires is taken from standard tables. These values of reactance and resistance are COD:- verted into voltage drop per amp. and then into Ipercent voltage drop per amp. at any arbitrary selected voltage. Entering the standard Mershon chart in the standard handbook using various values of power factors and the above values of percentage reactance and percentage resistance drops, values are obtained directly of the percent impedance voltage drop per it. per amp. at the various power factors. From these values, the curves upon the main body portion of the slide 3 which correspond to the various wire sizes of conductors may be plotted. The selection of constant Wire size curves is purely arbitrary as other variables could be used in "plotting the curves. *Fc-r exampleythe wire size indicia could have been placed at the side of the window 6 and the curves plotted as lines of constant power factor.

In setting the indicia upon the slide, the voltage scales at the top and bottom o'f the windowi'l, the current scale 1, the distance scale and the percent voltage drop scale 9 are all plotted upon the same logarithmic scale which isarbitrarily selected. The use of logarithmic scales is purely arbitrary for convenience and the'calculator can be arranged with indicia of other scales, such as arithmetic scales. The location ofithe-ivoltage scale at the'top of the-windowd is arbitrarily placed. Then the docation of the voltage scale at the bottom of the window 4 is determined by the relation between single. phase an'd three phase voltages. The currentscale 7 is aiibitrarilyzplaced upon the slide 2 in a. generally central location upon the slide :so that .the indicia-will appear within the window 4 :in the normal mcvementeof the slide. The indicia of theidistance in ft. OT'SCZJGiBLBlQ arbitrarilysplaced upon the scale 2 in a generally central location forithe range of values desired and have 'no -par- 'ticular relation "to either the :current :scale 1 nor the? power factor scale. a reverse logarithmic. scale s'o'as tozse-curc-wider spacings and-more accurate readings in the upper values of power factor. Any other form of scale forlthe power factor would simplyhave-changed the shape of the-wirezsize curve 'onthe main body portion: of :the .slidev3.

With the scales previously described set-.up'on -the=slides, the=lccation of'the curves-of constant iv'Jirei size-areplottedrdirectly'upon the main body portion of the slide from the values computed as previously described.

:fIn the use" of the calculator, the valued the circuit amper'es is-setiopposite the, proper. system volt-age. ln thep'osition of the slides shown, it is assumed that-the circuit current is 25 amperes and the circuit voltage 120=V0lts on azthreeaphase system. The indicia for 25 am'peres is set opposite 120 volta three phase as :shoWn, .thus-.determining the position of the slide 2. Let us then assume that the'pewer' factor ofthe circuit is"80% an'd the system uses No.-l=wire. The slide 3 is then moved to intersect the 'curvefor No. 10 wire *with 'the indicia at 80% 'power'i'actor in the position'shown. Assuming that thewiring distance one way infeet is .290 ft, the percent-voltage drop r'n'ay bereacl opposite the indicia for 200 ft. on the percentyoltage-drop scale. Thisindi-cates, for the example taken, a-voltage drop of-6.'7%. The calculator can, of "course'be utilized inother ways, depending on the known and unknown factors. .For example, in the example given, 'if 'a"per'cent drop of 6.7 is taken as'permissible, then for .varying .:percentages of power factor.

"unknown; .for example, if allowable wire: size and percentage drop are known, the allowable maxi mum distance in feet may be computed.

It is also to be understood that while the curves have been ploted for various wire sizes, they might be plotted just as well for other of the six variable sfactors; for example, the wire size might .lrlave beenplacedzin the form of indicia upon the bodyportionl and the curves might be drawn Similarly, the relative positions of the variables upon --the-calculator may be varied and the calculator may assume any mechanical form desired rather than .the slide form. shown; :for example, ;it :may bein. the form\-of rotating discs.

InEigure-Z-is shown a slightly modifiedrform of the calculator whichutilizes amain bod-yflportion I 4 within which are disposed the slides i2 and. The body portion H is provided with windows l4 and i5 and with a large transparentxwindow"l6. The indicia as to-system voltage and-currentmare similar-to those previously describediin connectionwith the preferred form of theinventionithe system voltages again being disposedabove and below the edges of window I l-and the circuit current indicia i! being disposed on the :slide :lhto be visible through the window 14. In :this embodiment, on the lowenportion .of theslidedi is disposed the arrow of indicator latoygivea datum point. Upon the-upper portion of the slide .4-3 are disposed the indiciaof the wiredistancesgat i9. The main portion of the slide 13 is provided withlogarithmic:indicia indicating the percentage voltage drop. The wiresize curves are :heredrawn directly upon-the transparent window ,2 I 8 rec ithat they are nowstationary. vThe power :factor per centages are placed on the face of-thebodygportion l l and have indicia lines crossing the transparent window 16. In-using thecalculator ;of.Figure 2 the same=gxample -.will be taken. Theslide .-wil1:be fixed in position byplacingthegcurrent indiciarlt'viopposite theithrec phase system voltage of 120 volts. This locates the positionofithe datumzarrow or indicator I8. The slide .1-3 is then moved to place theproperivalue of the wire length oppositethe indicator I 8, ingthis :example the indicia for 200 feet .being-,placed:opposite-the indicator. Then'the horizontal line ionpower factor is followed to its intersection with the curve for No. 10 wire. -Reading..,thro.ughithe transparent window it will be seenathat-this point .ofintersection occurs at .a value .of.;=,percentagevoltage drop of'6.7.%.

It :is. seen that-according .to the -.cal-cul-ator of this invention-all six .of the major variables vin the computation of the systemwcharacter-istios may-be directlyasetz and/or readby manipulation .ofthe calculator sothatitds unnecessarytocorrect, from tables. for variationin thepower factor of a'circultbut that by .the calculator the percent voltage drop, the-Wires'ize.or-otheryarh ablesmay be directly read !from the calculator with the actual circuit .power factor takeninto effect. This obviously 'facilitatesan'd accelerates the determining of the unknown quantity'in a system.

.While certain preferred embodiments of the invention have "been specifically disclosed, it is understood "that-the invention is "not limited thereto, as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

What is claimed is:

1. In a voltage drop, wire size calculator, stationary and movable elements, system voltage and circuit current indicia disposed on the stationary and a movable element and determinin the location of the movable element when the proper current indicia is disposed opposite the proper voltage indicia, a second movable element, Wire length and percentage voltage drop indicia on said first and second movable elements disposed in opposed relation, percentage power factor and Wire size indicia on said second movable element and said stationary element, the one of said last mentioned pair of indicia located on the movable element being in the form of a set of curves, one for each of a plurality of different values of the variable, the calculator by the setting of its movable elements permitting the reading of percentage voltage drop or wire size for the proper value of percentage power factor of the circuit.

2. In a voltage drop, wire size calculator, stationary and movable elements, system voltage and circuit current indicia disposed on the stationary and a movable element and determining the location of the movable element when the proper current indicia is disposed opposite the proper voltage indicia, a second movable element, Wire length and percentage voltage drop indicia on said first and second movable elements disposed in opposed relation, wire size indicia disposed on said second movable element, said wire WALLACE T. ALLEN.

REFERENCE S CIT ED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,552,113 Clarke Sept. 1, 1925 1,560,599 Moler Nov. 10, 1925 FOREIGN PATENTS Number Country Date 27,683 Great Britain Nov. 21, 1907 7,269 Great Britain Feb. 24, 1916 OTHER REFERENCES Pages 12, 13, 16, 17, 18, 20, 21, 31 and 32 of Special Slide Rules, by J. N. Arnold (Purdue University Engineering Bulletin No. 32), published in 1932.

Page 389 of Industrial Management for Dec. 1917; and page 44 of the Jan. 1918 issue thereof; published by The Engineering Magazine 00., No. 6 E. 39th St., New York; also pages 42 and 43 of the January 1918 issue of same publication.

Certificate of Correction Patent No. 2,443,882. June 22, 1948.

WALLACE T. ALLEN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 2, line 42, in the formule,

for 7 14.1 reed 741.1; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 14th day of September, A. D. 1948.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,443,882. June 22, 1948. WALLACE T. ALLEN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 2, line 42, in the formula,

for 714.1 read 741.1; and that the said Letters Patent should be read with this ggection therein that the same may conform to the record of the case in the Patent Signed and sealed this 14th day of September, A. D. 1948.

THOMAS E. MURPHY,

Assistant Commissioner of Patents. 

